Blockage removal and permeability enhancement method for coalbed methane wells by using electric pulses

ABSTRACT

A blockage removal and permeability enhancement method for coalbed methane wells by using electric pulses is applicable to high-efficiency exploitation of coalbed methane wells. The blockage removal and permeability enhancement method includes: constructing a positive electrode coalbed methane wellbore and a negative electrode coalbed methane wellbore from the ground to a coal bed; when the gas yield declines because cracks in the coal bed are closed gradually or blocked by granular impurities as coalbed methane extraction goes on after hydrofracturing, injecting a conductive ion solution into the positive electrode coalbed methane wellbore to fill the coal bed between the positive electrode coalbed methane wellbore and the negative electrode coalbed methane wellbore with the conductive ion solution; placing a positive electrode and a negative electrode downwards to predetermined permeability enhancement portions of the coal bed in the positive electrode coalbed methane wellbore and the negative electrode coalbed methane wellbore respectively; and breaking down, by using high-voltage electric pulses discharge, the coal bed filled with the conductive plasma solution between the positive electrode and the negative electrode, where shock waves generated from a large amount of energy act on the coal bed to cause the closed cracks in the coal bed to be opened again and extend and to remove the granules blocking the cracks, such that the number of the cracks in the coal bed is effectively increased and crack connectivity is improved.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to a blockage removal and permeability enhancement method by using electric pulses, and in particular, to a blockage removal and permeability enhancement method by using electric pulses which is applicable to coalbed methane wells with low permeability.

Description of Related Art

Coalbed methane, as a kind of clean energy, has great potential for exploitation. However, most coal beds in China feature low permeability. The existence of problems such as poor permeability, low exploitation efficiency and high exploitation cost of coal beds seriously limits the exploitation and utilization of coalbed methane in China. Hydrofracturing is the most commonly used technical means in current coalbed methane exploitation. However, the conventional hydrofracturing technique produces a small number of cracks in a coal bed, and the cracks extend in a small range. Therefore, the overall fracturing effect is undesirable, which finally results in low yield of a coalbed methane well. Meanwhile, the cracks in the coal bed are closed gradually or blocked by granular impurities as coalbed methane extraction goes on, and thus the extraction efficiency of the coalbed methane well is further reduced.

In recent years, high-power electric pulses techniques have been developed rapidly, and in China, some researches are made on methods for enhancing permeability of reservoirs by using high-power electric pulses techniques. For example, in Patent Publication No. CN104061014A entitled “Method for Increasing Yield of Coalbed Methane Wells by Using High-Power Electric Detonation Assisted Hydrofracturing”, a discharge electrode is powered by a high-voltage pulses power source and breaks down a water medium to form shock waves in water, and the shock waves act on the surrounding coal body to cause cracks therein. However, traveling in the form of spherical waves, the shock waves attenuate fast, resulting in high energy consumption and low efficiency. The method obtains a small effective fracturing range. The existing permeability enhancement techniques for coal beds by using electric pulses have problems such as high risks due to an excessively high pulses voltage, high energy consumption of shock waves, a small fracturing range, and low efficiency.

SUMMARY OF THE INVENTION

Technical problem: an objective of the present invention is to eliminate the defects in the prior art and provide a blockage removal and permeability enhancement method for coalbed methane wells by using electric pulses, which is simple, capable of removing coalbed blockage, low in energy consumption, and high in efficiency.

Technical solution: the blockage removal and permeability enhancement method for coalbed methane wells by using electric pulses according to the present invention includes the following steps:

a. constructing a positive electrode coalbed methane wellbore and a negative electrode coalbed methane wellbore from the ground to a coal bed, carrying out hydrofracturing in the positive electrode coalbed methane wellbore and the negative electrode coalbed methane wellbore according to conventional techniques, and performing coalbed methane extraction in the positive electrode coalbed methane wellbore and the negative electrode coalbed methane wellbore after hydrofracturing is completed;

b. when the gas yield declines after three months of coalbed methane extraction in the positive electrode coalbed methane wellbore and the negative electrode coalbed methane wellbore, arranging a conductive ion solution pumping station near the positive electrode coalbed methane wellbore, and arranging, in the positive electrode coalbed methane wellbore, a conductive ion solution conveying pipe connected to the conductive ion solution pumping station; injecting a high-voltage conductive ion solution into the positive electrode coalbed methane wellbore through the conductive ion solution pumping station; when the conductive ion solution is detected in the negative electrode coalbed methane wellbore, stopping injecting the high-voltage conductive ion solution into the positive electrode coalbed methane wellbore; arranging derricks at the opening of the two wellbores, and arranging a balancing support between the two wellbores;

c. placing, by using the derrick, a platform installed with a positive electrode and a high-voltage electric pulses generator downwards to a predetermined permeability enhancement portion of the coal bed in the positive electrode coalbed methane wellbore, and placing, by using the derrick, a platform installed with a negative electrode downwards to a predetermined permeability enhancement portion of the coal bed in the negative electrode coalbed methane wellbore;

d. adjusting, by using the balancing support, the positions of the platforms in the positive electrode coalbed methane wellbore and the negative electrode coalbed methane wellbore, such that the positive electrode and the negative electrode installed on the platforms in the two wellbores are in close contact with wellbore walls respectively, and the positive electrode and the negative electrode are arranged face to face on the same level;

e. turning on a high-voltage power source to charge the high-voltage pulses generator through a cable, where upon reaching a set discharge voltage, the high-voltage pulses generator discharges electricity to the coal bed between the positive electrode and the negative electrode, such that the closed cracks in the coal bed are opened again and extend under shock waves caused by discharge, the shock waves also have a shearing effect on a porous medium of the coal bed, and clay binders on the surface of coal particles are shaken off, thereby removing blockage in the coal bed;

f. after multiple times of discharge, turning off the high-voltage power source, moving the platform installed with the positive electrode and the high-voltage pulses generator out of the positive electrode coalbed methane wellbore, moving the platform installed with the negative electrode out of the negative electrode coalbed methane wellbore, and continuing to perform coalbed methane extraction in the positive electrode coalbed methane wellbore and the negative electrode coalbed methane wellbore; and

g. when the extraction yield of the coalbed methane declines, repeating Steps c-f to carry out electric pulses discharge and coalbed methane extraction for multiple times.

The high-voltage pulses generator has a discharge frequency of 10 to 60 Hz and a voltage range of 300 to 9000 kV.

A distance between the positive electrode coalbed methane wellbore and the negative electrode coalbed methane wellbore is 200 to 1500 m.

The conductive ion solution pumping station is capable of outputting the high-voltage conductive ion solution within a pressure range of 30 to 300 MPa.

The number of the multiple times of discharge is 15 to 100.

Beneficial effects: The present invention is applicable to high-efficiency exploitation of coalbed methane wells. A coal bed filled with a conductive plasma solution between a positive electrode and a negative electrode is broken down by using high-voltage electric pulses discharge. Shock waves generated from a large amount of energy act on the coal bed to cause closed cracks in the coal bed to be opened again and extend and to remove granules blocking the cracks, such that the number of the cracks in the coal bed is effectively increased and crack connectivity is improved. Coalbed methane extraction is performed based on hydrofracturing. When the extraction yield declines, the conductive plasma solution is added, the coal bed filled with the conductive plasma solution between the positive electrode and the negative electrode is broken down by using high-voltage electric pulses discharge, and shock waves of higher energy are produced. The closed cracks in the coal bed are opened again and extend under the shock waves, the shock waves also have a shearing effect on porous media of the coal bed, and clay binders on the surface of coal particles are shaken off, thereby solving the problem of blockage in the coal bed. The objective of blockage removal and permeability enhancement can be achieved by using pulses repeatedly. Compared with the prior art, the present invention has the following advantages:

(1) Based on conventional fracturing, when the coalbed methane yield declines, the coal body is broken down by using electric pulses repeatedly, such that the number of cracks in the coal bed is increased and the blockage in the cracks is removed, to maintain high yield of the coalbed methane well for a long time.

(2) After conventional fracturing, the coal body is broken down by using electric pulses, instead of using a large amount of fracturing fluid. Therefore, the waste and pollution of water resources can be reduced, and this technique is particularly applicable in drought areas.

(3) By injecting the conductive ion solution into the cracks formed by conventional fracturing, the conductivity of the coal bed can be increased, and the breakdown voltage for the coal bed between the positive electrode and the negative electrode is reduced, such that the coal body can be broken down with a low voltage, and the cost of electric pulses breakdown is lowered.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a blockage removal and permeability enhancement system for coalbed methane wells by using electric pulses according to the present invention.

In the drawing: 1: coal bed, 2: positive electrode coalbed methane wellbore, 3: negative electrode coalbed methane wellbore, 4: conductive ion solution pumping station, 5: conductive ion solution conveying pipe, 6: positive electrode, 7: negative electrode, 8: high-voltage electric pulses generator, 9: platform, 10: derrick, 11: balancing support, 12: high-voltage power source, 13: cable.

DETAILED DESCRIPTION OF THE EMBODIMENTS

An embodiment of the present invention is further described below with reference to the accompanying drawing.

As shown in FIG. 1, a blockage removal and permeability enhancement method for coalbed methane wells by using electric pulses according to the present invention includes the following steps:

The specific steps of the blockage removal and permeability enhancement method for coalbed methane wells by using electric pulses according to the present invention are as follows:

a. constructing two coalbed methane wellbores: a positive electrode coalbed methane wellbore 2 and a negative electrode coalbed methane wellbore 3, from the ground to a coal bed 1, carrying out hydrofracturing in the positive electrode coalbed methane wellbore 2 and the negative electrode coalbed methane wellbore 3 by using conventional techniques, connecting a coalbed methane extraction pipeline after hydrofracturing is completed, and performing coalbed methane extraction in the positive electrode coalbed methane wellbore 2 and the negative electrode coalbed methane wellbore 3;

b. when the gas yield declines after three months of coalbed methane extraction in the positive electrode coalbed methane wellbore 2 and the negative electrode coalbed methane wellbore 3, arranging a conductive ion solution pumping station 4 near the positive electrode coalbed methane wellbore 2, and arranging, in the positive electrode coalbed methane wellbore 2, a conductive ion solution conveying pipe 5 connected to the conductive ion solution pumping station 4; injecting a high-voltage conductive ion solution into the positive electrode coalbed methane wellbore 2 through the conductive ion solution pumping station 4; when the conductive ion solution is detected in the negative electrode coalbed methane wellbore 3, stopping injecting the high-voltage conductive ion solution into the positive electrode coalbed methane wellbore 2; arranging derricks 10 at the opening of the two wellbores, and arranging a balancing support 11 between the two wellbores, where a distance between the positive electrode coalbed methane wellbore 2 and the negative electrode coalbed methane wellbore 3 is 200 to 1500 m; and the conductive ion solution pumping station is capable of outputting the high-voltage conductive ion solution within a pressure range of 30 to 300 MPa;

c. placing, by using the derrick 10, a platform 9 installed with a positive electrode 6 and a high-voltage electric pulses generator 8 downwards to a predetermined permeability enhancement portion of the coal bed 1 in the positive electrode coalbed methane wellbore 2, and placing, by using the derrick 10, a platform 9 installed with a negative electrode 7 downwards to a predetermined permeability enhancement portion of the coal bed 1 in the negative electrode coalbed methane wellbore 3;

d. adjusting, by using the balancing support 11, the positions of the platforms 9 in the positive electrode coalbed methane wellbore 2 and the negative electrode coalbed methane wellbore 3, such that the positive electrode 6 and the negative electrode 7 installed on the platforms 9 in the two wellbores are in close contact with wellbore walls respectively, and the positive electrode 6 and the negative electrode 7 are arranged face to face on the same level;

e. turning on a high-voltage power source 12 to charge the high-voltage pulses generator 8 through a cable 13, where upon reaching a set discharge voltage, the high-voltage pulses generator 8 discharges electricity to the coal bed between the positive electrode 6 and the negative electrode 7, such that the closed cracks in the coal bed are opened again and extend under shock waves caused by discharge, the shock waves also have a shearing effect on a porous medium of the coal bed, and clay binders on the surface of coal particles are shaken off, thereby removing blockage in the coal bed; the high-voltage pulses generator 8 has a discharge frequency of 10 to 60 Hz and a voltage range of 300 to 9000 kV;

f. after 15 to 100 times of discharge, turning off the high-voltage power source 12, moving the platform 9 installed with the positive electrode 6 and the high-voltage pulses generator 8 out of the positive electrode coalbed methane wellbore 2, moving the platform 9 installed with the negative electrode 7 out of the negative electrode coalbed methane wellbore 3, and continuing to perform coalbed methane extraction in the positive electrode coalbed methane wellbore 2 and the negative electrode coalbed methane wellbore 3; and

g. when the extraction yield of the coalbed methane declines, repeating Steps c-f to carry out electric pulses discharge and coalbed methane extraction for multiple times. 

What is claimed is:
 1. A blockage removal and permeability enhancement method for coalbed methane wells by using electric pulses, comprising the following steps: a. constructing a positive electrode coalbed methane wellbore (2) and a negative electrode coalbed methane wellbore (3) from the ground to a coal bed (1), carrying out hydrofracturing in the positive electrode coalbed methane wellbore (2) and the negative electrode coalbed methane wellbore (3) according to conventional techniques, and performing coalbed methane extraction in the positive electrode coalbed methane wellbore (2) and the negative electrode coalbed methane wellbore (3) after hydrofracturing is completed; b. when the gas yield declines after three months of coalbed methane extraction in the positive electrode coalbed methane wellbore (2) and the negative electrode coalbed methane wellbore (3), arranging a conductive ion solution pumping station (4) near the positive electrode coalbed methane wellbore (2), and arranging, in the positive electrode coalbed methane wellbore (2), a conductive ion solution conveying pipe (5) connected to the conductive ion solution pumping station (4); injecting a high-voltage conductive ion solution into the positive electrode coalbed methane wellbore (2) through the conductive ion solution pumping station (4): when the conductive ion solution is detected in the negative electrode coalbed methane wellbore (3), stopping injecting the high-voltage conductive ion solution into the positive electrode coalbed methane wellbore (2); arranging derricks (10) at the opening of the two wellbores, and arranging a balancing support (11) between the two wellbores; c. placing, by using the derrick (10), a platform (9) installed with a positive electrode (6) and a high-voltage electric pulses generator (8) downwards to a predetermined permeability enhancement portion of the coal bed (1) in the positive electrode coalbed methane wellbore (2), and placing, by using the derrick (10), a platform (9) installed with a negative electrode (7) downwards to a predetermined permeability enhancement portion of the coal bed (1) in the negative electrode coalbed methane wellbore (3); d. adjusting, by using the balancing support (11), the positions of the platforms (9) in the positive electrode coalbed methane wellbore (2) and the negative electrode coalbed methane wellbore (3), such that the positive electrode (6) and the negative electrode (7) installed on the platforms (9) in the two wellbores are in close contact with wellbore walls respectively, and the positive electrode (6) and the negative electrode (7) are arranged face to face on the same level; e. turning on a high-voltage power source (12) to charge the high-voltage pulses generator (8) through a cable (13), wherein upon reaching a set discharge voltage, the high-voltage pulses generator (8) discharges electricity to the coal bed between the positive electrode (6) and the negative electrode (7), such that the closed cracks in the coal bed are opened again and extend under shock waves caused by discharge, the shock waves also have a shearing effect on a porous medium of the coal bed, and clay binders on the surface of coal particles are shaken off, thereby removing blockage in the coal bed; f. after multiple times of discharge, turning off the high-voltage power source (12), moving the platform (9) installed with the positive electrode (6) and the high-voltage pulses generator (8) out of the positive electrode coalbed methane wellbore (2), moving the platform (9) installed with the negative electrode (7) out of the negative electrode coalbed methane wellbore (3), and continuing to perform coalbed methane extraction in the positive electrode coalbed methane wellbore (2) and the negative electrode coalbed methane wellbore (3); and g. when the extraction yield of the coalbed methane declines, repeating Steps c-f to carry out electric pulses discharge and coalbed methane extraction for multiple times.
 2. The blockage removal and permeability enhancement method for coalbed methane wells by using electric pulses according to claim 1, wherein the high-voltage pulses generator (8) has a discharge frequency of 10 to 60 Hz and a voltage range of 300 to 9000 kV.
 3. The blockage removal and permeability enhancement method for coalbed methane wells by using electric pulses according to claim 1, wherein a distance between the positive electrode coalbed methane wellbore (2) and the negative electrode coalbed methane wellbore (3) is 200 to 1500 m.
 4. The blockage removal and permeability enhancement method for coalbed methane wells by using electric pulses according to claim 1, wherein the conductive ion solution pumping station is capable of outputting the high-voltage conductive ion solution within a pressure range of 30 to 300 MPa.
 5. The blockage removal and permeability enhancement method for coalbed methane wells by using electric pulses according to claim 1, wherein the number of the multiple times of discharge is 15 to
 100. 